I picked up a weird lens recently. Basically, it's a Tamron adaptall 2 300mm lens which seems to have had the original fitting removed and a simple tubular mount put in its place.

The protruding bit is 41mm diameter and about 19-20mm long, the bit at the end that looks like a screw fitting is actually just a groove in the tube. My guess is that it's supposed to click into some sort of quick-change mount, maybe for a cine camera or something of the sort. It'd be nice to know before I try to sell it.

Certainly an odd one, I've never seen anything like it.
Doesn't look as though it would be too difficult to convert back to the right mount providing you have another Tamron lens to get the mount from, probably wouldn't have to be to be exactly the same lens. Hopefully the aperture linkage hasn't been altered.
Was a very good lens in it's day and would still be now if only you could fit it on a camera..

Already done! But the lens I took the mount from is also rather nice, a 35-80 2.8-3.8 macro, and I can't make up my mind which one to sell as Adaptall, and which to sell with the mystery mount. Or I could just sell one and hope another Adaptall comes along...